1. FIELD OF THE INVENTION
The present invention relates to the field of earth boring tools, and more particularly to core catchers used in coring tools for catching or retaining a cut core.
2. DESCRIPTION OF THE PRIOR ART
The primary means for positively determining the nature of the rock formation which is being cut in a petroluem drilling operation remains cutting and retrieving an actual core cut from the formation. Traditionally, a core is cut using a coring bit which is a toroidally shaped bit having a cylindrical inner gage. Therefore, a cylindrical core is generally cut from the rock formation by the coring bit which is coupled to a drill collar. The core is then disposed within a coring tool as the coring bit continues to drill into the rock formation. Since hydraulic fluid is being forced downwardly within the drill string and collar to provide a cooling and cleaning action across the coring bit face, the core is isolated within the coring tool by being disposed within an inner barrel or tube. In addition thereto, the out core may be placed within a plastic sleeve within the inner barrel in order to facilitate removal of the core from the inner barrel and transportation of the core after retrieval from the bore hole. In order to successfully cut the core, it must of course easily slip fit through the core bit and into the inner barrel and sleeve. By the same token, once cut from the rock formation, the core, unless jammed within the core barrel, will just as easily slip out. In addition, some means is required in order to seize the core in order to break it from the formation from which, if in a consolidated rock formation, it integrally extends.
In response to this problem, the prior art devised a meas for gripping and retaining the core within the core barrel, namely a core catcher. In its most basic sense, prior art core catchers are comprised of cylindrical segments disposed within the coring tool and typically bearing against the inside surface of the coring bit or shoe. The inside surface of the coring bit or shoe, and the outside surface of the core catcher, are typically inclined with respect to the longitudinal axis of the drill string and coring bit. Thus, the surface upon which the core catcher rides is generally defined by the inside surface of a frustoconical shape. Two prior art methods are used to create a jamming force to prevent the core from leaving the core barrel. In the most common method, a split cylindrical catcher is designed to have an interference fit with the core, allowing the core to pass through the catcher by opening it slightly. Upward movement of the catcher is limited by the design, and downward movement causes a wedging of the core catcher against the core which will permit core breakage and retrieval. The other method uses gravity to initiate the wedging.
In the first method, i.e., split ring, the interference required to make the catcher first expand around and seize the core is a major cause of core blockage, especially when fractured formations are cut. Core blockage prevents the core from entering the barrel, causing the coring opertion to be terminated.
In the second method, i.e., gravity actuated core catchers, as highly deviated holes and horizontal drilling becomes more common, it is becoming increasingly necessary to not rely on gravity to initiate the wedging action.
Therefore, what is needed is some means by which a core catcher can be devised which will reliably and uniformly apply a grasping and retaining force to a consolidated core.